1. Field of the Invention
The invention relates to an end portion structure for connecting leads of a flexible printed circuit board, particularly to an end portion structure for connecting leads of a flexible printed circuit board in which, when a plurality of leads formed on a base film are connected to a plurality of terminal portions of an electronic part of an ink jet printer by brazing, brazing failure can be prevented and insulating performance is promoted.
2. Description of Related Art
Conventionally, in an ink jet printer, recording letters or images on record paper is done by ejecting ink from a plurality of injection nozzles installed on a recording head. A head holder is provided on a carriage movable in a recording direction and an ink jet mechanism comprising a recording head opposed to the record paper and a connecting cylinder (so-called manifold) forming an ink feed path communicating with the recording head is installed at the head holder. An ink cartridge storing ink for feeding to the recording head and provided with an ink feed port for fitting to the connecting cylinder is mounted attachably and detachably thereon.
For example, Japanese Unexamined Patent Publication No. 92-182138 discloses a recording head for an ink jet printer. The ink jet recording head is installed to the ink jet printer and has a central piezoelectric substrate in which a plurality of ink feed paths are formed, ink feed tubes for feeding ink to the respective ink feed paths are formed therein and leads, comprising copper wires, each connected to an electrode mounted to the piezoelectric substrate at the respective ink feed paths. End portions of a plurality of leads, formed on a base film of a flexible printed circuit board, are connected to terminal portions of the plurality of leads formed in the central piezoelectric substrate by brazing, for example, by soldering. The piezoelectric elements are deformed by drive signals supplied via the flexible printed circuit board by which ink is ejected and images are recorded.
In the case where the end portions of the plurality of leads of the flexible printed circuit board are connected to the terminal portions of the plurality of leads of the central piezoelectric substrate provided in the recording head by soldering, as shown in FIG. 6, to downsize the recording head a line width and a pitch between leads of the plurality of end portions of leads 122A are rendered as small as, for example, 0.5 mm or less which is defined as a fine pitch. Accordingly, an electrolytically-plated solder 130 is thinly coated on the plurality of end portions of leads 122A. Flux (so-called resin or rosin) promoting the soldering is coated thereon. The flexible printed circuit board 120 is positioned at and mounted on terminal portions 116A of the leads of the central piezoelectric substrate 114 and the plurality of end portions of leads 122A are soldered all at once by pressing and simultaneously heating at a predetermined temperature.
As stated above, in the case where the end portions of the leads formed on the flexible printed circuit board are connected to the terminal portions of the plurality of leads of the central piezoelectric substrate by soldering, as shown in FIG. 6, the electrolytically-plated solder coated on the end portions of leads 122A of the flexible printed circuit board 120 is molten as is the flux. The flux is obtained by dissolving the resin in alcohol and, accordingly, when the flux is heated to high temperatures (for example, approximately 300.degree. C.), gases are generated from the resin and alcohol is explosively evaporated instantaneously.
Because the thickness of each lead is as thin as approximately 18 through 35 .mu.m and the pitch between leads is defined as a fine pitch and is as small as approximately 0.5 mm or less, a connecting space S surrounded by the two lead connecting portions in each of which the end portion of lead wire 122A and the terminal portion 116A overlap and by the two base films 121 and 115, is very small. Therefore, gas generated by the evaporation of the alcohol or gasses generated from the resin cannot escape to the atmosphere and are confined to the very small connecting portion spaces S. As a result, the gas pressure increases and the plating solder 130 boils. At this moment, the boiled liquid plated solder 130 scatters so that a so-called solder bridge 130A spanning the gap between two adjacent lead connecting portions is formed or a plurality of solder balls 130B are scattered into space S. That is, the two lead end portions 122A are short-circuited by the formed solder bridge 130A or the solder balls 130B which absorb moisture and corrode degrading the insulating performance between the two leads.